Electrophotographic superlattice photoreceptor

ABSTRACT

An electrophotographic photoreceptor is constituted by forming a barrier layer, a photoconductive layer, and a surface layer on an aluminum substrate drum, in the order stated. The photoconductive layer of this electrophotographic photoreceptor has a superlattice layer of a hetero-junction constituted by alternately stacking thin a-Si:H layers and thin μc-SiC:H layers. In this superlattice structure, the optical bandgaps of the respective layers differ from each other, a-Si:H serves as a potential barrier, and μc-SiC:H serves as a potential well. When light is radiated on the photoconductive layer, a large number of photocarriers are generated. The photocarriers having one polarity pass through the barrier by way of a tunneling effect, and propagate in the photoconductive layer.

BACKGROUND OF THE INVENTION

The present invention relates to an electrophotographic photoreceptor for use in electrophotography.

Amorphous silicon containing hydrogen H (to be referred to as a-Si:H hereinafter) has received a great deal of attention as a photoconductive material, and has been used in a variety of applications, such as solar cells, thin film transistors, image sensors, and electrophotographic photoreceptors.

The materials used as the photoconductive layers in conventional electrophotographic photoreceptors can be categorized as either inorganic (e.g., CdS, ZnO, Se, or Se-Te) or organic (poly-N-vinylcarbazole (PVCZ) or trinitrofluorene). The a-Si:H has many advantages over the above-mentioned conventional organic and inorganic materials, such as that it is non-toxic and does not require recovery, high spectral sensitivity in the range of visible light is guaranteed, and its high surface hardness ensures high resistance to wear, and good anti-impact properties. For this reason, a-Si:H is receiving a great deal of attention as a promising electrophotographic photoreceptor.

The a-Si:H material has been developed as an electrophotographic photoreceptor on the basis of the Carlson system. In this case, good photoreceptor properties mean high dark resistance and high sensitivity to light. However, it is difficult to incorporate these two properties in a signal layer photoreceptor. A barrier layer is arranged between the photoconductive layer and a conductive support, and a surface charge-retaining layer is formed on the photoconductive layer, to constitute a multilayer structure, thereby satisfying the two requirements described above.

The a-Si:H material for use as a photoreceptor is prepared by glow discharge decomposition, using a silane gas. During the fabrication process, hydrogen is incorporated in the a-Si:H film, whereby the electrical and optical characteristics thereof are changed greatly, according to the change in hydrogen content. As the amount of hydrogen incorporated in the a-Si:H film increases, the film's optical bandgap widens and its resistance increases. Along with the increase in resistance, the sensitivity to long-wavelength light is degraded. Therefore, it is difficult to use such an a-Si:H film in a laser beam printer utilizing a semiconductor laser. When the content of hydrogen in the a-Si:H film is high, as described above, most of the components in the film can have bonding structures, such as those in (SiH₂)n and SiH₂, depending on film formation conditions. In this case, the number of voids and hence, the number of silicon dangling bonds, increases, thereby degrading the photoconductive characteristics of the film. Under these circumstances, the film cannot be used as an electrophotographic photoreceptor. However, when the content of hydrogen in the a-Si:H film is low, the optical bandgap is narrow and the resistance lower. As a result, the sensitivity to short-wavelength light is increased. A small content of hydrogen causes bonding of hydrogen atoms with the silicon dangling bonds, thus reducing the number of the silicon dangling bonds. For this reason, the mobility of photocarriers is degraded, thereby shortening their lifetime. At the same time, the photoconductive property of the film is degraded and the film cannot be used as an electrophotographic photoreceptor.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an electrophotographic photoreceptor wherein a large number of photocarriers can be generated upon radiation, the lifetime of the photocarriers can be long, a wide sensitivity range form visible light range to a near-infrared range can be guaranteed, and good environmental resistance properties can be ensured.

An electrophotographic photoreceptor according to a first aspect of this invention comprises: a conductive substrate, and a photoconductive layer, disposed on the substrate, for generating photocarriers upon radiation of light, the photoconductive layer including silicon and having a superlattice structure portion of alternately stacked thin amorphous semiconductor layers and thin microcrystalline semiconductor layers containing carbon.

An electrophotographic photoreceptor according to a second aspect of this invention comprises: a conductive layer, and a photoconductive layer, disposed on the substrate, for generating photocarriers upon radiation of light, the photoconductive layer including silicon and having a superlattice portion of alternately stacked thin amorphous semiconductor layers containing carbon and thin microcrystalline semiconductor layers.

An electrophotographic photoreceptor according to a third aspect of this invention comprises: a conductive substrate; and a photoconductive layer, disposed on the substrate, for generating photocarriers upon radiation of light, the photoconductive layer including silicon and having superlattice portion of alternately stacking thin amorphous semiconductor layers containing carbon and thin microcrystalline semiconductor layers, and the degree of crystallization of the thin microcrystalline semiconductor layers varying in the direction of thickness thereof.

In the electrophotographic photoreceptor of this invention, at least a portion of the photoconductive layer is constituted by alternately stacking thin layers made of the materials described above, and the stacked materials have optical bandgaps different from each other, thereby forming a superlattice structure at this stacked portion.

The superlattice structure has a periodical potential barrier wherein a layer having a larger optical bandgap serves as a barrier with reference to a layer having a smaller optical bandgap. In the superlattice structure, since a barrier layer is very thin, photocarriers pass through the barrier by way of a tunneling effect, and propagate in the superlattice structure. In addition, the number of photocarriers generated upon radiation of light is very large in such a super-lattice structure. For this reason, a lifetime of each photocarrier generated in this area is long and the mobility thereof is high, thereby significantly improving the sensitivity of the electrophotographic photoreceptor. Furthermore, by varying the degree of crystallization of a thin microcrystalline semiconductor layer, which is one of the thin layers constituting the superlattice structure, in the direction of thickness, the adhesion property between the thin layers can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained, as the same become better understood by reference to the following detailed description, when considered in connection with the accompanying drawings, in which:

FIG. 1 is a sectional view of an electrophotographic photoreceptor according to an embodiment of the present invention;

FIG. 2 is a sectional view of an electrophotographic photoreceptor according to another embodiment of the present invention;

FIG. 3 is a sectional view showing part of FIGS. 1 and 2 in an enlarged scale;

FIGS. 4A and 4B are views respectively showing an energy band of the superlattice structure;

FIGS. 5A to 5E are views respectively showing a state in which a degree of crystallization of a thin microcrystalline semiconductor layer is varied;

FIG. 6 is a schematic view of an energy gap of a photoreceptor; and

FIG. 7 is a view of an apparatus for manufacturing an electrophotographic photoreceptor of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will now be described in detail with reference to the accompanying drawings. FIG. 1 is a sectional view of an electrophotographic photoreceptor according to an embodiment of the present invention.

Conductive substrate 11 normally consists of aluminum and has a drum-like shape. Barrier layer 12 is formed on substrate 11, and photoconductive layer 13 for constituting a superlattice structure to be described later is formed on layer 12. Surface layer 14 is formed on layer 13 which generates photocarriers upon incidence of light. Of the generated photocarriers, those having one polarity are neutralized with electric charges on the photoreceptor surface, and the others having the other polarity propagate in layer 13 to substrate 11.

FIG. 2 is a sectional view of an electrophotographic photoreceptor according to another embodiment of the present invention.

In this electrophotographic photoreceptor, photoconductive layer 20 is constituted by charge-transporting layer 21 formed on barrier layer 12 and charge-generating layer 22 formed on layer 21, and hence is of a function separating type. Other layers of the electrophotographic photoreceptor are constituted similarly to those of the electrophotographic photoreceptor shown in FIG. 1. In layer 20, layer 22 constitutes a superlattice structure. In the photoreceptor shown in FIG. 2, photocarriers are generated in layer 22 upon incidence of light, and photocarriers having one polarity propagate in layer 21 and reach substrate 11.

In the embodiments shown in FIGS. 1 and 2, each of layers 13 and 22 is constituted by alternately stacking thin layers 31 and 32 and has a heterojunction superlattice structure, as shown in a sectional view of FIG. 3 in an enlarged scale. Layers 31 and 32 have optical bandgaps different from each other and a thickness falling within a range of 30 to 500 Å. By stacking such thin layers, a quantum size effect can be obtained. If the thin layer exceeds 500 Å, only a potential modulation effect caused by stacking can be obtained, and its property becomes an averaged one of the respective layers. Therefore, it is difficult to obtain the above effect.

Layer 31 consists of amorphous silicon containing hydrogen (to be referred to as a-Si:H hereinafter) or a-Si:H containing carbon (to be referred to as a-SiC:H), and layer 32 consists of microcrystalline silicon containing hydrogen and carbon (to be referred to as μc-SiC:H hereinafter). In this case, a carbon content in a-SiC:H and μc-SiC:H is preferably 0.2 to 40 at %.

0.01 to 30 at % of and, more preferably, 1 to 25 at % of hydrogen is preferably added to a-Si:H, a-SiC:H, and μc-SiC:H. Then, silicon dangling bonds can be compensated for, and dark and bright resistances can be balanced with each other, thereby improving the photoconductive property.

In accordance with the following physical characteristic, μc-Si:H is clearly distinguished from a-Si:H and polycrystalline silicon. That is, in an X-ray diffraction instrument, since a-Si:H is amorphous, only halo appears and no diffraction pattern can be found. On the other hand, μc-Si:H exhibits a crystal diffraction pattern obtained when 2θ is around 27° to 28.5°. In addition, μc-Si:H can be adjusted to have a dark resistance of 10¹¹ Ω·cm or more while polycrystalline silicon has a dark resistance of 10⁶ Ω·cm. This μc-Si:H consists of a phase mixture of microcrystals having a grain diameter of about several tens Å and amorphous phase.

In this invention, μc-SiC:H obtained by adding carbon to such μc-Si:H is used as a material for one of the layers constituting the superlattice structure.

As described above, when μc-SiC:H is used as part of layer 13 or 22, a dark resistance can be increased to improve the photoconductive property. C serves as a terminator in a silicon dangling bond to decrease a density of states present in a forbidden band between bands. For this reason, a dark resistance is assumed to be increased.

Similar to a-Si:H, μc-Si:H can be used to form layer 12 on substrate 11 by glow discharge decomposition using a silane gas as a source gas. In this case, if a temperature of the substrate and a high-frequency electric power are set higher than those in the case of forming a-Si:H, μc-Si:H can be formed more easily. In addition, when the substrate temperature and the high-frequency electric power are set higher, a flow rate of a source gas such as a silane gas can be increased, so that a film formation speed can be increased. Furthermore, when a gas obtained by diluting a high-order silane group gas such as SiH₄ and Si₂ H₆ as a source gas by hydrogen, μc-Si:H can be formed more efficiently. Note that hydrocarbon such as methane may be added as a carbon source to the above materials to fabricate μc-SiC:H.

On the other hand, a silicon halide gas such as an SiF₄ gas can be used as a source gas. In addition, μc-Si:H and a-Si:H can be similarly fabricated using a gas mixture of a silane gas and a silicon halide gas. Note that these thin layers can be formed by physical methods such as sputtering instead of glow discharge decomposition. A hydrogen or helium gas can be used as a silane carrier gas as needed.

In addition, when the conductivity types of thin layers of a-Si:H, a-SiC:H, and μc-SiC:H are set to be p- or n-type, flow of charges from substrate 11 to the photoconductive layer can be prevented. In order to set the conductivity types of a-Si:H, a-SiC:H, and μc-SiC:H to be p-type, elements belonging to Group III of the Periodic Table, e.g., boron B, aluminum Al, gallium Ga, indium In, and thallium Tl are doped. On the other hand, in order to set the conductivity types of a-Si:H, a-SiC:H and μc-SiC:H to be n-type, elements belonging to Group V of the Periodic Table, e.g., nitrogen N, phosphorus P, arsenic As, antimony Sb, and bismuth Bi are doped.

In the superlattice structure shown in FIG. 3, when a-Si:H having an optical bandgap of 1.75 eV or a-SiC:H having an optical bandgap of 1.8 eV is used as layer 31 and μc-SiC:H having an optical bandgap of 1.6 eV or 1.65 eV is used as layer 32, the energy band becomes as shown in FIG. 4A or 4B. In this case, a-Si:H or a-SiC:H having a larger optical bandgap serves as a potential barrier, and μc-SiC:H having a smaller optical bandgap serves as a potential well.

As described above, by stacking layers 31 and 32 having optical bandgaps different from each other, a superlattice structure having a periodic potential barrier wherein a layer having a larger optical bandgap serves as a barrier with reference to a layer having a smaller optical bandgap is formed regardless of a size of the optical bandgap. In this superlattice structure, since a barrier layer is very thin, photocarriers pass through the barrier and propagate in the superlattice structure by a tunneling effect. In addition, in such a superlattice structure, the number of photocarriers generated upon incidence of light is large, so that high sensitivity to light can be obtained. Note that by varying the bandgap and the thickness of the thin layers constituting the superlattice structure, an apparent bandgap of a layer having a heterojunction superlattice structure can be arbitrarily adjusted.

When layers 31 and 32 are respectively formed by a-SiC:H and μc-SiC:H, in order to improve a contact property between thin layers, a degree of crystallization of each thin μc-SiC:H layer constituting layers 13 and 22 can be changed in the direction of thickness. FIG. 5A shows an example in which a degree of crystallization of μc-SiC:H is substantially linearly decreased in the direction of thickness. FIG. 5B shows an example in which a degree of crystallization of μc-SiC:H is sequentially decreased in the direction of thickness. FIG. 5C shows an example in which a degree of crystallization of μc-SiC:H is substantially linearly increased in the direction of thickness. FIG. 5D shows an example in which a degree of crystallization of μc-SiC:H is sequentially increased in the direction of thickness. FIG. 5E shows an example in which a degree of crystallization of μc-SiC:H is maintained constant to an intermediate point and then sequentially increased in the direction of thickness. It is a matter of course that a degree of crystallization of a-SiC:H is 0 in either case. A degree of crystallization of μc-SiC:H is preferably changed within a range of 50 to 90%.

By restricting flow of charges between substrate 11 and layer 13 or 22, layer 12 improves a charge-retaining function on the photoreceptor surface, thereby improving charging capacity of the photoreceptor.

Layer 12 may be formed by a-Si:H or μc-SiC:H.

Regardless whether layer 12 is formed by μc-SiC:H or a-Si:H, when the photoreceptor surface is to be positively charged by the Carlson method, layer 12 is formed to have p-type so as to prevent injection of electrons from substrate 11 to the photoconductive layer. On the other hand, when the photoconductive surface is to be negatively charged, layer 12 is formed to have n-type so as to prevent injection of holes from substrate 11 to the photoconductive layer. In addition, a material having a high insulating property may be used as layer 12.

In order to form layer 12 to have p- or n-type, an impurity similar to that used to form the photoconductive layer to have p- or n-type may be doped. In addition, in order to form layer 12 to have a high insulating property, carbon, nitrogen, and/or oxygen may be added.

The thickness of layer 12 is preferably from 100 Å to 10 μm.

Surface layer 14 is formed on layer 13 or 22. The μc-SiC:H constituting layer 13 or 22 has a relatively large refractive index of 3 to 3.4, so that reflection of light tends to occur on the surface thereof. When reflection of light occurs, an amount of light to be absorbed in layer 13 or 22 is decreased to increase loss of light. Therefore, layer 14 is preferably formed to prevent reflection of light. Layer 14 also serves to prevent damage to layer 13 or 22. In addition, layer 14 improves the charging capacity so that charges are sufficiently retained on the surface.

Examples of materials for forming layer 14 are inorganic materials such as a-SiN:H (amorphous silicon containing nitrogen and hydrogen), a-SiO:H (amorphous silicon containing oxygen and hydrogen), and a-SiC:H (amorphous silicon containing carbon and hydrogen), and organic materials such as polyvinyl chloride and polyamide.

As for charge-transporting layer 21 shown in FIG. 2, a material such as a-Si:H is used to obtain a good contact property with respect to layers 12 and 22. In this case, oxygen, carbon, and/or nitrogen are preferably added to increase resistance, thereby improving charge-retaining capacity on the surface.

When the surface of the electrophotographic photoreceptor having the above arrangement is charged with a positive voltage of about 500 V by corona discharge, a potential barrier as shown in FIG. 6 is formed in the case of the electrophotographic photoreceptor having a function separating photoconductive layer as shown in, e.g., FIG. 2. When light (hν) is incident on the photoreceptor, photocarriers, i.e., electrons and holes are generated in the superlattice structure of layer 22. The electrons in the conductive layer are accelerated toward layer 14 by an electric field in the photoreceptor, and the holes are accelerated toward substrate 11. In this case, the number of photocarriers generated on the boundary between thin layers having different optical bandgaps is much larger than that of the carriers generated in the bulk. For this reason, sensitivity to light is high in this superlattice structure. In addition, in a potential well layer, a lifetime of photocarriers is increased 5 to 10 times that obtained by a single layer not having a superlattice structure because of a quantum effect. Furthermore, in the superlattice structure, a periodic barrier layer is formed due to discontinuity of the bandgap. However, the photocarriers can easily pass through the barrier layer by a tunneling effect, so that effective mobility of photocarriers is equivalent to that obtained in the bulk, thereby obtaining a good propagating property of photocarriers.

As has been described above, according to the superlattice structure obtained by stacking thin layers having optical bandgaps different from each other, a high photoconductive property can be obtained, and an image clearer than that obtained by a conventional photoreceptor can be obtained. In addition, when the superlattice is formed by a-SiC:H and μc-SiC:H, a degree of crystallization of the μc-SiC:H layer can be varied in the direction of thickness to improve the contact property between the thin layers.

FIG. 7 shows an apparatus for manufacturing an electrophotographic photoreceptor according to the present invention, utilizing the glow discharge method. Gas cylinders 41, 42, 43, and 44 store source gases such as SiH₄, B₂ H₆, H₂, and CH₄. Gases in cylinders 41, 42, 43, and 44 can be supplied to mixer 48, through flow control valves 46 and pipes 47 respectively. Each cylinder has pressure gage 45. The operator controls each valve 46 while monitoring corresponding pressure gage 45, thereby controlling the flow rate of each gas and their mixing ratio. The gas mixture is supplied from mixer 48 to reaction chamber 49. Rotating shaft 10 extends vertically extends from bottom 11 of reaction chamber 49, and can be rotated about the vertical axis. Disk-like support table 52 is fixed on the upper end of shaft 50 such that the surface of table 52 is perpendicular to shaft 50. Cylindrical electrode 53 is arranged inside chamber 49 such a that the axis of electrode 53 is aligned with the axis of shaft 50. Drum-like substrate 54 for a photoreceptor is placed on table 52 such that the axis of the former is aligned with the axis of shaft 50. Drum-like substrate heater 55 is arranged inside substrate 54. RF power source 56 is connected between electrode 53 and substrate 54, and supplies an RF current therebetween. Rotating shaft 50 is driven by motor 58. The internal pressure of reaction chamber 49 is monitored by pressure gage 57, and chamber 49 is connected to a proper evacuating means, such as a vacuum pump, through gate valve 58.

In order to manufacture a photoreceptor in the apparatus having the construction described above, drum-like substrate 14 is placed in reaction chamber 49, and gate valve 59 is opened to evacuate chamber 49 to a vacuum of about 0.1 Torr or less. The predetermined gases from cylinders 41, 42, 43, and 44 are supplied to chamber 49, at a predetermined mixing ratio. In this case, the flow rates of the gases supplied to chamber 49 are determined such that the internal pressure of chamber 49 is set to be 0.1 to 1 Torr. Motor 58 is operated to rotate substrate 54. Substrate 54 is heated to a predetermined temperature by heater 55, and an RF current is supplied between electrode 53 and substrate 14, thereby generating a glow discharge therebetween. An a-Si:H layer is deposited on substrate 54. N₂ O, NH₃, NO₂, N₂, CH₄, C₂ H₄, and O₂ gases and the like may be added to the feed gas to add the element N, C, or 0 in the a-Si:H layer.

As is apparent from the above description, the electrophotographic photoreceptor according to the present invention can be manufactured in a closed-system manufacturing apparatus, thus guaranteeing the safety of the operators. Since the electrophotographic photoreceptor has high resistance to heat, to humidity, and to wear, repeated use thereof does not result in degradation; thus, a long service life is assured.

Electrophotographic photoreceptors according to the present invention were formed, and their electrophotographic characteristics were tested in the following manner.

EXAMPLE 1

An aluminum drum substrate having a diameter of 80 mm and a length of 350 mm and subjected to oxygen, alkali, and sandblast treatments as needed to prevent interference, was mounted in a reaction chamber, and the interior of the reaction chamber was exhausted by a diffusion pump (not shown) to obtain a vacuum pressure of about 10⁻⁵ Torr. Thereafter, the drum substrate was heated to a temperature of 250° C. and rotated at 10 rpm, and an SiH₄ gas with a flow rate of 500 SCCM, a B₂ H₆ gas with a ratio of flow rate of 10⁻⁶ with respect to the SiH₄ gas, and a CH₄ gas with a flow rate of 100 SCCM were supplied into the reaction chamber, so that the interior of the reaction chamber was adjusted to be 1 Torr. Then, a high-frequency electric power of 13.56 MHz was applied to an electrode to generate plasma of SiH₄, B₂ H₆, and CH₄ between the electrode and the substrate, thereby forming a barrier layer consisting of p-type a-SiC:H.

Then, the B₂ H₆ /SiH₄ ratio was set to be 10⁻⁷, and a high-frequency electric power of 500 W was applied to form a 20-μm charge-transporting layer consisting of i-type a-Si:H.

Thereafter, discharge was temporarily stopped, and the SiH₄, CH₄, and H₂ gases, respectively with flow rates of 100, 5, and 1,200 SCCM were supplied into the reaction chamber to adjust the reaction pressure to be 1.8 Torr. Then, the high-frequency electric power of was applied to form a 50-Å thin μc-SiC:H layer. Thereafter, flow rates of the SiH₄ and H₂ gases were set to be 500 and 750 SCCM, respectively, the flow rate of the B₂ H₆ gas was set to obtain the B₂ H₆ /SiH₄ ratio of 10⁻⁷, and high-frequency electric power of 500 W was applied to form a 50-Å thin a-Si:H layer. The above operation was repeated to alternately stack 250 thin μc-SiC:H layers and 250 thin a-Si:H layers, thereby forming a 5-μm charge-generating layer with a heterojunction superlattice structure. Then, a 0.1-μm surface layer of a-SiC:H was formed. Note that a degree of crystallization and a crystal grain diameter of the thin μc-SiC:H layer were respectively 60% and 30Å.

When the photoreceptor surface thus formed was positively charged at about 500 V and exposed to white light, the light was absorbed in the charge-generating layer, and photocarriers of electron-hole pairs were generated. In this test example, a large number of photocarriers were generated, and a long lifetime and high propagating property of photocarriers were obtained. As a result, a clear image of high quality was obtained. In addition, when the photoreceptor manufactured in this test example was repeatedly charged, a transferred image was proved to have very good reproducibility and stability and superior durabilities such as high resistance to corona, humidity, and wear. Furthermore, the photoreceptor thus manufactured has a high sensitivity to light having a long wavelength of 780 to 790 nm which is an oscillation wavelength of a semiconductor laser. When the photoreceptor was mounted in a semiconductor laser printer to form an image by the Carlson process, a clear image was obtained with high resolution even when an exposure amount of the photoreceptor was 25 erg/cm². Note that even when the photoreceptor was exposed to normal light and repeatedly charged as in the case of the white light, a transferred image had good reproducibility and stability and was superior in durabilities such as high resistance to corona, humidity, and wear.

EXAMPLE 2

In this example, an arrangement of layers was the same as that in Example 1 except that the barrier layer was p-type μc-SiC:H. In this case, flow rates of the SiH₄, H₂, and CH₄ gases were set to be 100, 1,500, and 20 SCCM, respectively, and a flow rate of the B₂ H₆ gas was set to obtain a B₂ H₆ /SiH₄ ratio of 1×10⁻². Then, the above gases were supplied into the reaction chamber, and the high-frequency electric power of 1.2 KW was applied at a reaction pressure of 1.5 Torr, thereby forming a 0.5-μm thin layer. A degree of crystallization and a crystal grain diameter of the layer were 65% and 70 Å, respectively.

When the electrophotographic photoreceptor thus obtained was mounted in a semiconductor laser printer and an image was formed by the Carlson process, a clear image was obtained with high resolution in both cases. In addition, when the photoreceptor was repeatedly charged, an image was proved to have good reproducibility and stability superior in durabilities such as high resistance to corona, humidity, and wear.

EXAMPLE 3

In this example, an arrangement of layers was the same as that in Example 1 except that a superlattice constituting a charge-generating layer consisted of μc-SiC:H nd a-SiC:H. In this case, after formation of a charge-transporting layer, discharge was temporarily stopped, the SiH₄, CH₄, and H₂ gases with flow rates of 100, 5, and 1,200 SCCM, respectively were supplied into the reaction chamber, and the reaction pressure was adjusted to be 1.8 Torr. Thereafter, the high-frequency electric power of 1.2 KW was applied to form a 50-A thin μc-SiC:H layer. Then, flow rates of the SiH₄, H₂, and CH₄ gases were set to be 500, 750, and 10 SCCM, respectively, a flow rate of the B₂ H₆ gas was set to obtain a B₂ H₆ /SiH₄ ratioi of 10⁻⁷, and the high-frequency electric power of 500 W was applied to form a 50-Å thin a-SiC:H layer. The above operation was repeated to alternately stack 250 thin μc-SiC:H layers and 250 thin a-SiC:H layers, thereby forming a 5-μm charge-generating layer of a heterojunction superlattice structure. Note that a degree of crystallization and a crystal grain diameter of the thin μc-SiC:H layer were 60% and 50 Å, respectively.

When the photoreceptor surface thus formed was positively charged at about 500 V and exposed to white light, the same good characteristics as those in Examples 1 and 2 were obtained.

EXAMPLE 4

In this example, an arrangement of layers was the same as that in Example 3 except that a barrier layer was p-type μc-SiC:H. In this case, film formation conditions and physical characteristics of the barrier layer were the same as those in Example 2.

According to the electrophotographic photoreceptor thus manufactured, the same good characteristics as those in Examples 1 to 3 were obtained.

EXAMPLE 5

In this example, an arrangement of layers was the same as that in Example 3 except that one of the layers of a superlattice constituting the charge-generating layer consisted of μc-SiC:H in which a degree of crystallization was varied in the direction of thickness. In this case, after formation of the charge-transporting layer, discharge was temporarily stopped, the SiH₄, methane, and hydrogen gases with flow rates of 100, 5, and 1,500 SCCM, respectively, were supplied, the pressure was adjusted to be 1.5 Torr, and the high-frequency electric power of KW was applied to form a 50-Å thin μc-SiC:H layer. Then, the silane, methane, and hydrogen gases respectively of 500, 1,000, and 7.5 SCCM were supplied to obtain a diborane/silane ratio of 5×10⁻⁷, and the high-frequency electric power of 500 W was applied at a pressure of 1 Torr, thereby forming a 250-Å thin a-SiC:H layer. Thereafter, 250 thin a-SiC:H layers were formed under the same conditions. In addition, a voltage to be applied was gradually decreased to form 250 thin μc-SiC:H layers. In this case, the final electric power was 1.0 KW. In this manner, a charge-generating layer of a heterojunction structure obtained by stacking the thin a-SiC:H and μ c-SiC:H layers was formed. Note that a degree of crystallization and a grain diameter of the thin μc-SiC:H layer were respectively 80% and 60 Å immediately after film formation, and were 55% and 40 Å after film formation.

When the photoreceptor surface thus obtained was positively charged at about 500 V and exposed to white light, the same good characteristics as those in Example 4 were obtained.

EXAMPLE 6

In this example, an arrangement of layers was the same as that in Example 5 except that the barrier layer was p-type μc-SiC:H. In this case, the film formation conditions and the physical characteristics of the barrier layer were the same as those in Examples 2 and 4.

According to the electrophotographic photoreceptor thus manufactured, the same good characteristics as in Examples 1 to 5 were obtained.

EXAMPLE 7

In this example, the film formation conditions of μc-SiC:H of the charge-generating layer in the electrophotographic photoreceptor having the same layer arrangement as that in Examples 5 and 6 was varied, i.e., a degree of crystallization was changed as shown in FIGS. 5A to 5E. As a result, an excellent image as that in Example 1 was obtained in both cases.

Note that in the above examples, a thickness of the charge-generating layer was 5 μm, but it is not limited to this value. For example, when the thickness is set to be, e.g., 1 or 3 μm, the photoreceptor can be similarly put to practical use.

In addition, although a-Si:H and a-SiC:H were used for one type of thin layers, it is a matter of course that materials are not limited to these.

Furthermore, the number of types of the thin films are not limited to two as in the above examples, but three or more types of thin layers may be stacked. More specifically, a boundary need only be formed between thin layers having optical bandgaps different from each other. 

What is claimed is:
 1. An electrophotographic photoreceptor comprising:a conductive substrate; and a photoconductive layer, disposed on said substrate, for generating photocarriers upon irradiation with light, said photoconductive layer including silicon and having a superlattice portion of alternately stacked first semiconductor layers comprised of amorphous silicon containing hydrogen and second semiconductor layers comprised of microcrystalline silicon containing hydrogen and carbon, each of said first and second semiconductor layers having a thickness of between about 3 and 500 Å.
 2. A photoreceptor according to claim 1, further comprising a barrier layer interposed between said substrate and said photoconductive layer, said barrier layer comprising at least one semiconductor layer selected from the group consisting of a layer comprised of amorphous silicon containing hydrogen and a layer comprised of microcrystalline silicon containing hydrogen.
 3. A photoreceptor according to claim 2, wherein said barrier layer contains an element belonging to Group III or V of the Periodic Table.
 4. A photoreceptor according to claim 2, wherein said barrier layer contains at least one element selected from the group consisting of carbon, oxygen, and nitrogen.
 5. A photoreceptor according to claim 1, further comprisinga surface layer disposed on said photoconductive layer.
 6. A photoreceptor according to claim 1, wherein said photoconductive layer contains an element belonging to Group III or V of the Periodic Table.
 7. An electrophotographic photoreceptor comprising:a conductive substrate; and a photoconductive layer, disposed on said substrate, for generating photocarriers upon irradiation with light, said photoconductive layer having a superlattice portion of alternately stacked first semiconductor layers comprised of amorphous silicon containing hydrogen and carbon and second semiconductor layers comprised of microcrystalline silicon containing hydrogen and carbon, each of said first and second semiconductor layers having a thickness of between about 30 and 500 Å.
 8. A photoreceptor according to claim 7, further comprising a barrier layer interposed between said substrate and said photoconductive layer, said barrier layer comprising at least one semiconductor layer selected from the group consisting of a layer comprised of amorphous silicon containing hydrogen and a layer comprised of microcrystalline silicon containing hydrogen.
 9. A photoreceptor according to claim 8, wherein said barrier layer contains an element belonging to Group III or V of the Periodic Table.
 10. A photoreceptor according to claim 8, wherein said barrier layer contains at least one element selected from the group consisting of carbon, oxygen, and nitrogen.
 11. A photoreceptor according to claim 8, further comprisinga surface layer disposed on said photoconductive layer.
 12. A photoreceptor according to claim 8, wherein said photoconductive layer contains an element belonging to Group III or V of the Periodic Table.
 13. An electrophotographic photoreceptor comprising:a conductive substrate; and a photoconductive layer, disposed on said substrate, for generating photocarriers upon irradiation with light, said photoconductive layer having a superlattice portion of alternately stacked first semiconductor layers comprised of amorphous silicon containing hydrogen and carbon and second semiconductor layers comprised of microcrystalline silicon containing hydrogen and carbon, wherein(i) each of said first and second semiconductor layers has a thickness of between about 30 and 500 Å and (ii) the degree of crystallization of each of said second semiconductor layers varies in the direction of thickness thereof.
 14. A photoreceptor according to claim 13, further comprising a barrier layer interposed between said substrate and said photoconductive layer, said barrier layer comprising at least one layer selected from the group consisting of a layer comprised of amorphous silicon containing hydrogen and a layer comprised of microcrystalline silicon containing hydrogen.
 15. A photoreceptor according to claim 14, wherein said barrier layer contains an element belonging to Group III or V of the Periodic Table.
 16. A photoreceptor according to claim 14, wherein said barrier layer contains at least one element selected from the group consisting of carbon, oxygen, and nitrogen.
 17. A photoreceptor according to claim 14, further comprisinga surface layer disposed on said photoconductive layer.
 18. A photoreceptor according to claim 14, wherein said photoconductive layer contains an element belonging to Group III or V of the Periodic Table.
 19. A photoreceptor according to claim 2, wherein said microcrystalline silicon containing hydrogen has (i) an x-ray diffraction pattern at 20 equal to about 27° to 28.5° and (ii) a dark resistance of about 10¹¹ Ω cm.
 20. A photoreceptor according to claim 8, wherein said microcrystalline silicon containing hydrogen has (i) an x-ray diffraction pattern at 20 equal to about 27° to 28.5° and (ii) a dark resistance of about 1011°8 cm.
 21. A photoreceptor according to claim 14, wherein said microcrystalline silicon containing hydrogen has (i) an x-ray diffraction pattern at 20 equal to about 27° to 28.5° and (ii) a dark resistance of about 10¹¹ Ω cm.
 22. A photoreceptor according to claim 5, wherein said surface layer comprised at least one member selected from the group consisting of amorphous silicon containing nitrogen and hydrogen; amorphous silicon containing oxygen and hydrogen; amorphous silicon containing carbon and hydrogen; polyvinyl chloride and polyamide.
 23. A photoreceptor according to claim 11, wherein said surface layer comprises at least one member selected from the group consisting of amorphous silicon containing nitrogen and hydrogen; amorphous silicon containing oxygen and hydrogen; amorphous silicon containing carbon and hydrogen; polyvinyl chloride and polyamide.
 24. A photoreceptor according to claim 17 wherein said surface layer comprises at least one member selected from the group consisting of amorphous silicon containing nitrogen and hydrogen; amorphous silicon containing oxygen and hydrogen; amorphous silicon containing carbon and hydrogen; polyvinyl chloride and polyamide. 